This invention relates to a planar antenna and, more particularly, to a planar antenna, referred to as a radial line slot antenna, which is excited by an axially symmetric transverse mode.
Radial line slot antennas are described in a variety of literature. For example, refer to "A Radial Line Slot Antenna for 12 GHz Satellite TV Reception" in IEEE TRANSACTIONS ON ANTENNA AND PROPAGATION, Vol. AP-33, No. 12, December 1985, pp. 1347-1353; "Characteristics of a Radial Line Slot Antenna for 12 GHz Band Satellite TV Reception" in IEEE TRANSACTIONS 0N ANTENNA AND PROPAGATION, Vol. AP-34, No. 10, October 1986, pp. 1269-1272; and "Slot Coupling in a Radial Line Slot Antenna for 12-GHz Band Satellite TV Reception" in IEEE TRANSACTIONS ON ANTENNA AND PROPAGATION, Vol. 36, No. 12, December 1988, pp. 1675-1680.
The planar antennas excited by an axially symmetric mode described in this literature all possess a double-layered structure having two propagation layers. Specifically, a radio wave from a feeder source is supplied to the center of the lower propagation layer, the wave is propagated radially outward along the lower propagation layer, guided to the upper propagation layer at the terminus or outer portion of the lower layer, propagated toward the center along the upper propagation layer and radiated by a number of slots in the process of propagating through the upper propagation layer. Circular polarization and linear polarization are decided by the arrangement of the slots. With this double-layered structure, radio waves propagate from the outer periphery toward the center at the radiating layer (namely the upper propagation layer) having the radiating slot surface. In a case where radio waves excited by an axially symmetric mode thus propagate from the outer periphery toward the center, an inner electromagnetic field f(r) is expressed as follows: ##EQU1## where A represents a proportional coefficient, k represents a propagation constant, r represents the radius, and .alpha. is a proportional coefficient of power radiated per unit length in the radial direction. The coefficient .alpha. is a positive value and is referred to as a "coupling factor".
On the other hand, aperture power distribution U(r) at the position of the radius is as follows: ##EQU2## where .alpha. is positive. Therefore, this is an arrangement in which it is theoretically easy to obtain an aperture power distribution that is nearly uniform in the radial direction.
Residual radio waves that are not radiated are absorbed by an absorber at the center. However, the sectional area in the traveling direction of the radio waves is small near the center, and therefore the amount of radio waves to be thus absorbed is small. As a result, the antenna is efficient.
However, this double-layered structure has a drawback, namely that manufacture is very difficult. Specifically, it is required that the plate material intervening between the upper and lower propagation layers be so held as not to impede propagation of the radio waves. In addition, it is required that the layer widths of the upper and lower propagation layers be maintained at predetermined values.
From the viewpoint of such manufacture, a single-layered structure in which radio waves are radiated in the course of propagating radially outward from the center is advantageous. When the antenna is excited with axial symmetry in such a single-layered structure, the fed radio waves propagate radially outward from the center and are radiated little by little in the course of such propagation. In a planar antenna excited in an axially symmetric mode, the specification of this application refers to an antenna in which the exciting radio waves propagate from the outer edge toward the center within a propagation layer having a radiating surface as being of the "outer-feed type" (or "outer-excitation type"), and refers to an antenna in which the excited radio waves propagate from the center toward the outer edge within the propagation layer as being of the "inner-feed type" (or "inner-excitation type"),
In the antenna of the inner-excitation type, the inner electromagnetic field f(r) within the waveguide is as follows: ##EQU3## which is the opposite of the two-layered structure mentioned above, namely the antenna of the outer-excitation type. Even if there is no radiation by means of the radiating slots (.alpha.=0), the electromagnetic field is very large at the center and weakens as the outer edge of the antenna is approached. Since there is radiation from the radiating slots (.alpha.&gt;0) in addition to the foregoing, the electromagnetic field weakens sharply the nearer the outer edge of the antenna. Accordingly, with an antenna of the inner-excitation type, it is considered to be very difficult in practice to establish a nearly uniform profile distribution in the radial direction.
Residual radio waves that are not radiated are absorbed at the outer peripheral surface in order to avoid reflection. However, in comparison with the antenna of the outer-feed type, the cross-sectional area is extremely large. Since this absorption becomes loss, it has been considered that, in theory, efficiency is very low in the antenna of the inner-excitation type. For these reasons, it has been thought to be difficult or impossible to obtain a highly efficient, practical planar antenna which uses the inner-excitation method. Another reason is that because of this, much more research has been devoted to planar antennas using the outer-feed method than those using the inner-feed method.